Long distance telephone calls are commonly routed over the Internet. While the Internet is fast, relatively inexpensive and usually reliable, it also presents special problems for voice applications.
For example, once a voice packet has been sent, the sender has no control over the path the packet will take. While multiple routes are typically available, due to various parameters and conditions of systems along each path, some paths will provide better performance than others, that is, some paths will be more efficient than others in delivering packets in timely fashion and with few lost packets.
These qualities are particularly important with voice applications because voice signals or messages, to be intelligible, place real-time constraints on the system. Packets must be delivered within reasonable times of each other so that the full voice message can be re-assembled at the receiving end. Furthermore, small noticeable delays are irritating to users, while larger delays make a system unusable.
RFC 1771 from the Internet Engineering Task Force (IETF) describes the Internet as a collection of arbitrarily connected Autonomous Systems (ASs). An AS is a set of routers under a single technical administration, using an interior gateway protocol and common metrics to route packets within the AS and using an exterior gateway protocol to route packets to other ASs.
FIG. 1 illustrates the components utilized in the end-to-end call where voice is transmitted over the internet.
A call begins at a device such as a telephone 10, which connects to a public switched telephone network (PSTN) 12. The PSTN 12 comprises many local exchange carriers (LEC). Here, the telephone or user terminal 10 is connected directly to local exchange carrier 14. The local exchange carrier 14 then connects to long distance carrier (IXC) 12. The long distance carrier 16 then has several options for delivering the voice signal to the destination terminal 44. One of these options is to transmit the voice over internet protocol.
To do so, the long distance carrier 12 connects to an internet source 18, which comprises a Point of Presence (POP) 20. Calls arriving at the POP 20 from various long distance carriers 16 arrive at a voice switch 22 within the POP 20. The switched voice signals are then converted to packets at gateway 34. These internet protocol (IP) packets are then sent to one of several routers or output ports depending on a routing policy and/or announced best routes. Each output port is linked to an autonomous systems (AS) 30, where an AS “is a set of routers under a single technical administration, using an interior gateway protocol and common metrics to route packets within the AS, and using an exterior gateway protocol to route packets to other ASes.” See IETF RFC 1930. Thus, each AS is operated by a “carrier” and each carrier may provide different services and charge its own rates.
While en route to a destination 32, the packets may traverse several autonomous systems 30. Each autonomous system encountered is considered a “hop.”
The packets then arrives, along with other packets from the same or other sources, at destination routers 28, where they are converted back to voice signals at the receiving POP 34 by a packet-to-voice gateway 36. The voice signals are routed within the POP 34 to voice switches 38 which route the voice channels to a public switched telephone network 42 to which the destination 44 is connected. The call is then routed to the destination terminal 44.